In vitro attachment of bilins to apophycocyanin. II. Determination of the structures of tryptic bilin peptides derived from the phycocyanobilin adduct.

In vitro reaction of phycocyanobilin (PCB) with apophycocyanin results in the specific addition of the bilin to two of the cysteinyl residues, alpha-Cys-84 and beta-Cys-82, which normally function in PCB attachment (Arciero, D. M., Bryant, D. A., and Glazer, A. N. (1988) J. Biol. Chem. 263, 18343-18349). These bilin binding sites are designated alpha-1 and beta-1, respectively. Tryptic digestion of the apophycocyanin-PCB adduct releases two major bilin peptides, alpha-1 mesobiliverdin (MBV) and beta-1 MBV, which encompass the two bilin-binding sites. These peptides were examined by 1H NMR and fast atom bombardment mass spectroscopies. The NMR spectra show that the bilin is attached to each peptide through a thioether linkage identical to the linkage observed in the corresponding tryptic peptides, alpha-1 PCB and beta-1 PCB, derived from the natural product, C-phycocyanin. However, the NMR spectra of the adduct peptides lack the resonances corresponding to protons at positions C2 and C3 of ring A seen in the spectra of the alpha-1 PCB and beta-1 PCB peptides. Fast atom bombardment mass spectroscopy shows the masses of the alpha-1 MBV and beta-1 MBV peptides to be 2 atomic mass units lower than those of the alpha-1 PCB and beta-1 PCB peptides, respectively. Comparison of the bilin portion of the NMR spectra of the alpha-1 MBV and beta-1 MBV peptides to the NMR spectra of PCB and mesobiliverdin confirms that the bilin of the two adduct peptides resembles mesobiliverdin in having an extra double bond in the C2-C3 position of ring A. These results show that the major bilin products arising from the reaction of PCB with apophycocyanin differ from the bilins present in C-phycocyanin. The relevance of these results to the biosynthetic pathway for the attachment of tetrapyrroles to phycobiliproteins is discussed.